Recently, in mobile communications, various information such as image and data in addition to audio are targets for information transmission. With this trend, demands for high-reliable, high-speed transmission are increased. However, when high-speed transmission is performed in mobile communication, the influence of delay waves due to multi-path cannot be avoided, and transmission performances degrade due to frequency selective fading.
As a technique for avoiding frequency selective fading, multicarrier communication represented by the OFDM (Orthogonal Frequency Division Multiplexing) scheme has attracted attention. The multicarrier communication is a technique of performing high-speed transmission by transmitting data using a plurality of subcarriers where the level of transmission speed is suppressed not to cause frequency selective fading. To be more specific, in the OFDM scheme, frequencies of a plurality of subcarriers in which data is mapped are orthogonal to each other, so that the OFDM scheme provides the highest frequency efficiency in multicarrier communications and can be realized by relatively simple hardware constitution. Therefore, the OFDM scheme has attracted attention as a communication method employed by cellular scheme mobile communications, and has been studied variously. Further, according to the OFDM scheme, to avoid ISI (Intersymbol Interference), the tail end part of an OFDM symbol is attached to the head of that of OFDM symbol as a CP (Cyclic Prefix). By this means, it is possible to prevent ISI as long as the delay time of delay waves stays within the time length of CP (hereinafter “CP length”).
On the other hand, recently, multicast communication has been studied. Multicast communication is not one-to-one communication such as unicast communication, but is one-to-many communication. That is, in multicast communication, one radio communication base station apparatus (hereinafter “base station”) transmits the same data to a plurality of radio communication mobile station apparatuses (hereinafter “mobile stations”) at the same time. By this multicast communication, in mobile communication systems, for example, delivery services of music data and video image data and broadcast services such as television broadcast are realized. Further, services using multicast communication are assumed to be services for relatively wide communication areas that cannot be covered by one base station, and, consequently, multicast communication covers wide communication areas entirely by transmitting the same data from a plurality of base stations. By this means, data for multicast communication (multicast data) is transmitted through a multicast channel shared by a plurality of base stations. Thus, in the multicast channel, the same data is transmitted from a plurality of base stations at the same time, and, consequently, a mobile station nearby the cell boundary receives mixed multicast data comprised of multiple multicast data from a plurality of base stations.
Here, if the OFDM scheme is employed in multicast communication, in a mobile station nearby the cell boundary, when a plurality of the same OFDM symbols transmitted from a plurality of base stations at the same time are received successively within time for the CP length, these OFDM symbols are combined and this combined OFDM symbol with amplified, received power is received. To compensate a channel fluctuation (phase fluctuation and amplitude fluctuation) of the combined signal by channel estimation, the channel estimation value of the combined signal is needed. Therefore, in multicast communication using the OFDM scheme, pilots used for estimating the channel estimation value need to be transmitted from a plurality of base stations at the same time, as in the case of multicast data.
On the other hand, in a unicast channel, a plurality of base stations transmit mutually different data (unicast data) (see Non-Patent Document 1). By this means, unicast data is multiplied by the scrambling code unique to each base station on a per cell basis such that unicast data from each of a plurality of base stations can be classified in a mobile station. By this means, according to unicast communication, pilots used for calculating a channel estimation value is multiplied by the scrambling code unique to each base station as in the case of unicast data.
Further, as shown in FIG. 1, studies are conducted for time-multiplexing unicast data and multicast data on a per subframe basis and switching between the unicast channel and the multicast channel over time upon use (see Non-Patent Document 2). This document discloses one frame formed with subframes #1 to #20, that is, with twenty subframes, as an example. Further, above-described pilot (P) that differs between unicast data and multicast data is time-multiplexed over the head of each subframe. Further, in FIG. 1, multicast data is multiplexed every three subframes, as an example of a frame structure.    Non-Patent Document 1: 3GPP RAN WG1 LTE Adhoc meeting (2005.06) R1-050589 “Pilot channel and scrambling code in evolved UTRA downlink”    Non-Patent Document 2: 3GPP RAN WG1 LTE Adhoc meeting (2005.06) R1-050590 “Physical channels and multiplexing in evolved UTRA downlink”